State highway geometric design manual cross section

STATE HIGHWAY GEOMETRIC DESIGN MANUALSECTION 6: CROSS SECTION

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Provisional Issue Draft: S6-Cross Section_7-3-02 Draft.wpd - 8 March 2002 (10:24AM)

6 Cross Section

6.1 Introduction6.1.1 GeneralThe cross section of a road is a vertical plane at right angles tothe road control line. It is viewed in the direction ofincreasing stationing and shows transverse detail of thevarious elements that make up the road's structure, sometimesfrom boundary to boundary. The main purpose of a crosssection is to show the variation of elements within the designand their interaction with the natural topography.

The cross section elements should be designed to ensure thatthe use of the space available within the road reserve issympathetic to the natural environment and user expectations,while maintaining a balance between construction,maintenance and operating (including crash) costs.

The width and crossfall of traffic lanes and shoulders arebased on traffic needs and drainage requirements.

The form of the remainder of the cross section, ie. the batterslopes of fills and cuts, depends on the type of material to beexcavated, environmental factors and the importance of theroad. The widths and slopes of the various cross sectionelements may be varied within acceptable limits to achieve abalanced, economical, functional and aesthetic result. Detailsof acceptable widths and slopes of elements, together withguidelines for selection of the appropriate values are given inthis section of the manual.

Some typical cross sections are illustrated in Figure 6.1

6.1.2 Important Cross Section Design FactorsThe three most important factors that need to be born in mindwhen using of this section of the Manual are:

• The Cross Section is Part of the Total Road DesignPackage

The cross section forms only a part of the total roaddesign. Decisions about the dimensions to be used foran individual cross section element are considerablyinter-dependent on other design considerations, ie. theshoulder width can only logically be set in relation tothe sight distance available due to vertical andhorizontal alignments, the pavement surface treatment,adjoining travel lane widths and predicted trafficvolumes and composition. A holistic approach musttherefore be taken with road design and the crosssection needs to be designed in conjunction with allother aspects of the road design, includinglandscaping.

• Relative Costs Must Always be Considered

For most roadworks the pavement and its wearingsurface is the most significant factor in the total cost ofthe project. It is, however, very important to ensurethat the width of pavement is appropriate for the road’spurpose and design requirements. Pavement materialsare expensive and small increases in the widths oftraffic lanes and shoulders can add significantly to thetotal cost of the project, even if the percentageincreases are relatively small.

Special care is needed in cases where improvementsare being made to roads on existing formations. Adopting dimensions that will require widening of theformation can cause a large increase in the cost of thework. However, once established on a project,marginal increases in dimensions may not represent asignificant increase in the total cost. The cost ofalternatives should always be examined to ensure thatthe most cost effective solution is adopted.

• The Clear Zone is an Essential Part of Cross SectionDesign

It is an unfortunate fact that vehicles do sometimes runoff the road. Shoulder, verge and batter design mustensure a clear zone which will allow an errant vehicleto traverse this area with minimum damage to itselfand occupants. The clear zone concept underlines thefact that a reasonably flat, well compacted andunobstructed road side environment is highly desirable,especially on high speed roads.

An appropriate speed related clear zone should beprovided in urban areas, especially on newconstruction works. Footpaths will usually provide anadequate clear zone provided utility poles, signsupports and heavy structures are kept to the rear of thefootpath, or made frangible, and all planting consists offrangible species. Undergrounding of utility serviceswill assist in keeping the footpath clear of obstructions.

6.1.3 The Road ReserveThe road reserve is measured between the property boundarieson each side of the road. It must be of sufficient width toaccommodate the ultimate planned traffic lanes, a medianwhen necessary, shoulders, footpaths, public utilities, drains,and the space necessary for the cut and batter slopes, includingany extra clearances necessary adjacent to high cut batters toprevent possible future erosion affecting adjacent properties.

A wide road reserve will permit the construction of gentleslopes which result in greater safety for motorists and enableseasier and more economical maintenance.


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(a): Two-lane Two-way Rural Road

(b): Dual Carriageway Rural Road

(c): Two-lane Urban Road

(d) Dual Carriageway Urban Road (½ section only - second one-way carriageway and adjacent verge are not shown)

Figure 6.1: Typical Cross Sections(More extensive cross section details are shown in Section 6.11)


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6.1.4 Cross Section DeterminationThe Flow Chart shown in Figure 6.2 details a procedure to help determine the most appropriate cross section to be used.References to other relevant sections of the Manual are given for assistance.

Figure 6.2: Cross Section Determination Flow Chart


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Figure 6.3: Minimum State Highway Sealed Widths

6.1.3 Minimum Seal Widths for State Highways


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6.2 Traffic Lanes6.2.1 GeneralA traffic lane is that part of a roadway reserved for the normalone way movement of a single stream of vehicles. Trafficlanes provide a variety of functions important to the overallefficient function of the road hierarchy, such as:

• through road,• special - bus, transit, etc.,• auxiliary (turning or overtaking),• parking,• cycling.

Traffic lane width is normally determined after considerationof the road's annual average daily traffic (AADT) and peakhour traffic volumes, where relevant. Vehicle dimensions andthe combination of speed and traffic volume should also betaken into account.

Lane width and road surface condition have a substantialinfluence on the safety and comfort of road users. In ruralareas the additional costs incurred in providing wider lanescan be partially offset by the reduction in long term pavementmaintenance costs resulting from heavy vehicle wear in thevicinity of the pavement edge on narrow lane roads.

Narrow lanes also force vehicles to travel laterally closer toone another than their drivers are normally comfortable with,particularly at higher travel speeds.

Drivers also tend to reduce their travel speed, or shift closerto the centre of the lane/road, or both, when they perceive ahazardous object is too close to either the nearside or offsideof their vehicle. The most common driver reaction to thistype of hazard is, however, a movement of their vehicle awayfrom the hazard. The offset of a fixed hazard from the edgeof the traffic lane beyond which this reaction is not observedis termed the 'Shy Line'. The shy line is normally taken as thedistance from the edge of the traffic lane to the outer edge ofshoulder, or the distance shown in Table 6.1, whichever is thegreater.

Design or

85th PercentileSpeed(km/h)

Shy Line Offset(m)

Nearside(Left)

Offside(Right)

# 70 1.5 1.0

80 2.0 1.0

90 2.5 1.5

$ 100 3.0 2.0

Table 6.1: Shy Line Offsets

Reductions in lane width reduces the lateral clearancebetween vehicles and also to fixed obstacles. This leads toreduced travel speed and lane capacity and Tables 6.2 and 6.3show the reduction in lane capacity caused by a fixed hazardclose to the road.

Clearanceto fixedobstacleclose tothe road

Lane Capacity(% of 3.5m lane capacity)

3.5 m lane 3.3 m lane 3.0 m lane 2.7 m lane

1.8 100 93 84 70

1.2 92 85 77 65

0.6 81 75 68 57

0.0 70 65 58 49

Table 6.2: Two-lane Two-way Road Lane Capacity

Clearanceto fixedobstacleclose tothe road

Lane Capacity(% of 3.5m lane capacity)

3.5 m lane 3.3 m lane 3.0 m lane 2.7 m lane

1.8 100 95 89 77

1.2 98 94 88 76

0.6 95 92 86 75

0.0 88 85 80 70

Table 6.3: Four-lane Dual Carriageway Road Lane Capacity

NOTES:1. The width of a lane adjacent to a kerb

excludes the width of the channel (if any).2. The legal width limit of heavy commercial

vehicles is 2.5 m and the majority ofheavy vehicles are built to this maximumwidth. An additional width of 200 mm isneeded on each side of these vehicles toaccommodate their wing mirrors.

ARRB Transport Research was commissioned to developminimum estimated lane width requirements for variousheavy vehicles. Data from this study is shown in thehistogram below.

A clearance component is not included in these estimatedvehicle path lane widths. Typically, 0.5 m needs to be addedto give a lane width sufficient for efficient traffic operations.


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These minimum vehicle path widths are for a straight, levelroad with an average NAASRA roughness of 120 counts/km,a crossfall of 4.5% and two test speeds. This particularcombination is regarded as too extreme for typical designconditions, but no other data is available at present. Also, adifferent set of test conditions would need to be consideredfor roads with other than straight travel paths. However, theresearch suggests that, in a speed environment of 90 km/h,most of New Zealand’s legal size heavy vehicles couldcomfortably operate on roads that have an effective lanewidth of 3.5 m. State highway lane widths should thereforeconform to the following general standards:

• The desirable minimum width of a traffic lane is3.5 m. Auxiliary lanes should also conform to thiswidth.

• On kerbed urban arterial roads the width of theleft-hand lane should be increased to 4.5 m, toaccommodate cyclists.

• Single one-way traffic lanes, such as freeway ramps,should be at least 4.5 m wide, to allow traffic to pass astationary parked, or broken down, vehicle.

In some situations, such as restricted road reserve width, theHighway Standards and Strategy Manager may approve adeparture from these standards. In these situations the lanewidths should be reduced by increments of 0.1 m until anacceptable solution, or 3.0 m, ie. the minimum state highwaylane width, is reached.

Figure 6.4: Lane Width Notation

6.2.2 Two-Lane Two-Way Rural RoadsThe minimum traffic lane width for a two-lane two-way ruralstate highway should be determined from Table 6.4. Wherethe AADT lies near to a boundary between groups the use ofthe higher value must be carefully considered.

Where the design speed in mountainous terrain exceeds80 km/h, or 100 km/h in undulating terrain, or where there isa high percentage of heavy vehicles (20% for 500 AADT and5% for 2000 AADT), a lane width of 3.5 m is desirable.

6.2.3 Multilane Rural Roads, Expressways andMotorways

The minimum traffic lane width for multilane rural roads,expressways and motorways is 3.5 m.

Desirably, any rural road consisting of four lanes or moreshould have a central median to separate the opposing trafficflows.

EffectiveWidth of

TwoTrafficLanes

(m)

Anticipated AADT at Opening

LowFutureGrowth(< 3%)

ReasonableFutureGrowth(3 - 6%)

HighFutureGrowth(> 6%)

6.6 up to 700 up to 500 up to 300

6.5 700 - 1700 500 - 1200 300 - 900

7.0 * over 1700 over 1200 over 900

* Where local conditions dictate widths in excess of 7.0 m may be considered

Table 6.4: Traffic Lane Width Guidelines forTwo-lane Two-way Roads

6.2.4 Urban RoadsThe desirable state highway traffic lane width in urban areasis 3.5 m. Where the road reserve width is restricted, lanewidth(s) may, with Standards and Strategy Manager approval,be reduced.

The differing functions and uses of each lane must be takeninto account when 'squeezing' an extra lane from an existingor partially widened road formation, or fitting the requirednumber of lanes into the space available, is necessary. Lanewidths must be allocated on an equitable basis in thesesituations and the widths varied in 0.1 m increments from thedesired 3.5 m to a minimum of 3.0 m, with the followingprovisions:

• On Straight Alignments: 3.l m is the minimum widthfor a kerbside lane. All other lanes must be at least3.0 m wide.

• On Curved Alignments: Widening in accordance withTable 6.5 must be applied.

Normal Lane Width(m)

Radius(m)

Widening(m per lane)

3.560 - 100 0.6

100 - 150 0.3

3.0 to 3.4

60 - 100 0.9

100 - 150 0.6

150 - 300 0.4

300 - 450 0.3

Table 6.5: Lane Widening on Curves in Urban Areas

It is desirable to locate a barrier kerb at least 0.5 m clear ofthe edge of the adjacent traffic lane, to compensate for adriver’s tendency to shy away from them. Usually, the widthof the channel will provide an adequate clearance.


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Where over-dimension vehicles use the road, eg. heavyhaulage by-passes, wharf access routes, etc, allowance mustto be made for the size of these vehicles and their trackingcharacteristics. The local heavy vehicle operators must beconsulted and a suitable design vehicle developed for theseroutes.

6.2.5 Bus RoutesThe desirable lane width for a bus route on new constructionprojects is 3.5. The minimum width is 3.0 m.

On existing roads the following conditions apply:

(a) Kerbside lanes used by buses should not be marked lessthan 3.0 m wide, as measured from the face of the kerb.Where a lane has to be 3.0 m or less in width thekerbside lane should be made wider than the adjacentlanes, to offset the effects of drivers shying away fromkerbs, channels, power poles and other roadsidestructures.

(b) Site specific measures to mitigate the effects of narrowlanes should be investigated. These include parkingrestrictions, median width variations, indented bus bays,etc.

(c) The appropriate bus operator must be consulted duringthe planning stages to ensure that the road designproposed is acceptable to them.

6.2.6 Auxiliary LanesAuxiliary lanes, other than parking and turning lanes, shouldhave the same width as the adjacent through traffic lanes

6.2.7 Parking Lanes(a) Parking lanes are not normally provided on rural roads

but provision for parking should be made at rest areasand layby’s.

(b) It is normal practice to provide parking lanes on urbanroads and.

The provision of parking lanes on urban roads, wherethey often serve the function of a shoulder, isdetermined mainly by the operational requirements ofthe road. These are described briefly below:

• Major Routes - Single Purpose: Limited accessurban routes and urban motorways which caterfor moving traffic and only the occasional stop. Design parameters are similar to those of ruralroads, shoulders are normally provided and therural cross section forms are acceptable. Parkingis not an issue other than providing for theoccasional stopped vehicle, eg. broken down,fatigue stop, etc.

• Major Routes - Mixed Purpose: These form thebulk of major urban routes. They normally havefrontage development and have to cater for siteaccess movements, parked vehicles, the parkingand un-parking of these vehicles as well asmoving traffic. Their design principles differfrom those of rural roads and their cross sections

usually include parking lanes, which often servethe function of shoulders, in addition to thetraffic lanes.

• Local Access Roads: The main design controlsfor these types of road are property access,property drainage and the width between kerbs. Providing for parked vehicles is also animportant factor.

• Frontage / Service Roads: These are not really aseparate road class and they may be either localaccess or mixed purpose roads. The lowest classof service road provides only local access, eg.residential access, industrial development,shopping centres, etc.On urban corridors in large cities they caneventually carry significant proportions ofthrough traffic. As the through trafficcomponent of a service road increases there is atendency for fewer connections to the rest of thestreet system and in these cases service roadstend to provide a mixed function service withincreasing importance given to other than localaccess traffic. They can eventually becomearterial / collector roads in their own right.

(c) Lane Width

• An exclusive parallel parking lane should have aminimum width of 2.5 m. Where kerb andchannel is used the width of the channel may beincluded in this width, although this is not adesirable practice. This minimum width shouldonly be used in situations where there is nolikelihood of the lane being required for trafficpurposes in the future and the reduced capacityof the road produced by this arrangement isadequate for the traffic volumes expected.

• A parallel parking lane which is used as a travellane during peak times should have the samewidth as a normal traffic lane, ie. desirably 3.5 mand 3.0 m minimum, as measured to the lip at thechannel.

• Shared parallel parking and traffic lanes shouldbe at least 5.5 m wide, ie. 3.5 m traffic lane plusa 2.0 m, as measured to the lip of the channel,parking lane. This is the borderline betweenacceptable and difficult traffic operations.

• In areas where frequent parking is combinedwith reasonable arterial traffic volumes all sparewidth should be put into the outer trafficlane/parking lane combination as this is wheremost of the 'side friction' occurs. This situationoccurs mainly in suburban shopping/businessareas on arterial roads. where speeds tend to beslower and there is merit in reducing the throughtraffic lane widths to obtain this additional space.

• Where angle parking is provided the AustroadsGuide to Traffic Engineering Practice, Part 11:Parking should be used to determine theappropriate parking lane width. Markingsshould be as defined in the Manual of TrafficSigns an Markings, Part II: Markings.


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6.2.8 Turning Lanes / Turning Roadways andOn / Off Ramps

The desirable width for a turning lane is 3.2 to 3.5 m. Theminimum width is 3.0 m.

On and off ramps lanes must be 3.5 m wide and the rampcarriageway should be at least 5.0 m, and never less than4.5 m, wide to ensure reasonable traffic flow conditions pasta stopped or broken down vehicle. Refer to drawing TNZ1/2000/77/7994/1 for the standard geometric designs formotorway on and off ramps.

Where a ramp has two or more lanes each lane must be atleast 3.5 m wide on a straight and widening should be appliedon curves.

6.2.9 Cycle LanesCyclists are legitimate users of public roads and provisionmust be made for them. They can be accommodated on or offthe carriageway within the road reserve or on a separate route.

Figure 6.5 shows the lateral forces which affect cyclists whenthey are in close proximity to heavy vehicles travelling atspeed.

Figure 6.5: Lateral Forces on Cyclists Caused byHeavy Vehicles

NOTE: Lateral forces may be increased where enclosedroadways create a 'wind tunnel' effect, eg. underbridges. In these cases, it may be appropriate toencourage cyclists to use the footpath, with fencingand/or signing, to increase the separation distance.

Where cycle lanes are provided on state highways theappropriate widths for shared and exclusive cycle lanesshould be determined from the Austroads Guide to TrafficEngineering Practice, Part 14: Bicycles. Exclusive cyclelanes are preferred where space is available and Table 6.6lists details of common Cycle Lane configurations.

Traffic lane widths that result in a 'squeeze point' for cyclistsshould be avoided wherever possible. On left hand curves,parked cars tend to 'truncate' the corner and use up the spaceallocated to cycles in a shared lane and parking restrictionsshould be considered in these situations.

Cycle lanes must be designed on the basis that bicycles havetyres only 20 mm wide and the appropriate facilities provided,eg. bicycle safe sump gratings. Also, all adjoining roadfurniture must be free from protrusions that may snag theclothing of cyclists, or cause unnecessary injury in a fall.

Speed(km/h)

FacilityLane Width

(m)

60 Exclusive lane 1.5



60 Shared with parking lane 4.0 *

80 Shared with parking lane 4.5

* 4.5 m is the preferred width, to allow cyclists toavoid a parked car door opening.

Table 6.6: On Road Cycle Lane Widths

Where a cycle lane is located adjacent to a concrete safetybarrier less than 1200 mm high, and it is considered necessaryto provide some additional protection to prevent cyclistsfalling over the barrier, eg. on bridges higher than five metres,a rail located 1400 mm above the surface of cycle lane shouldbe provided on top of the barrier. The rail should be aminimum of 75 mm diameter and must not have the potentialto spear an impacting vehicle.

Where a cycle lane located is adjacent to a steel guardrail insimilar conditions to those described above, a 1400 mm highwire mesh fence should be provided behind the guardrail.

Refer to Section 6.3.8 for more details of making provisionfor cyclists on road shoulders.

6.2.10 Location of Kerbs and/or ChannelsAs a general principle kerbs should be avoided in high speedenvironments. Where they are used the kerb and/or channelmust be located outside the traffic lanes on both the nearsideand the offside of the road.

Where the design speed is:

(a) >70 km/h: The kerb and/or channel face must beoffset at least 1.0 m from the edge of the adjacent trafficlane.

(b) ####70 km/h: A reduced kerb and/or channel offsetmay be used, ie. 0.5 m in areas without street lightingand zero (0.0 m) in areas with street lighting.


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6.3 Shoulders6.3.1 GeneralThe shoulder is that portion of the carriageway adjacent to thetraffic lanes and flush with the pavement surface. Itaccommodates stopped vehicles, provides lateral support tothe road pavement layers and, if sealed, offers improvedconditions for cyclists.

Shoulder width is measured from the edge of the traffic laneto the shoulder hinge point. Edge lines should therefore bemarked so that their inside edge corresponds to the outsideedge of the traffic lane.

All safety barriers, signs, guide posts, drains and kerbs mustbe located outside the shoulder.

Factors that need to be considered when determining shoulderwidths include:

• Support for the pavement structure: 0.5 m is theminimum width needed to provide a realistic level ofpavement support in most situations.

• Space for a driver to use to avoid a collision and regaincontrol: The shoulder will rarely be sufficiently wideenough for this purpose and the ‘clear zone’ is thearea in which it will occur. The wider the shoulder,the more use it will be for this purpose and, based onNorth American studies that have shown that theaccident risk is halved if the shoulder width isincreased from 0.6 to 3.0 m, a shoulder width of 3.0 mis desirable wherever it can be reasonably provided. .

• Clearance to posts and other fixed objects: Anadequate shoulder width provides shy line clearance. It also provides marginal increases in capacity but thiseffect reduces to zero when shoulder widths exceeds1.8 m, see Tables 6.1, 6.2 and 6.3 for more details.

• Space for a stationary vehicle to stop clear, or partlyclear, of the traffic lanes: A 2.5 m shoulder isdesirable to allow passenger cars to stop clear of thetraffic lanes. Wider shoulders, ie. 3.0 m or more,should be used on high standard high volume roads toprovide for larger vehicles to stop clear of the trafficlanes.

• Sight distance across the inside of a horizontal curve.• Costs of providing additional width, particularly where an

existing formation is being used: A Cost Benefitanalysis must be carried out if greater than standardwidths are proposed.

6.3.2 Two-Lane Two-Way Rural Roads(a) Shoulder Width

Table 6.7 shows shoulder the width requirements forrural roads that have minimal pedestrian and/or bicycletraffic. A transition taper of approximately 1:50 shouldbe used between different width shoulders that adjoinone another. These transition lengths may, however,need to be lengthened to ensure a satisfactoryappearance.

NominalShoulder

Width(m)

Situation

0.5 - 1.0Widths less than 1.0 m should only be used on low volumeroads (<500 vpd), pavement overlay and/or rehabilitationprojects where a full width carriageway seal is to be providedand formation widening is not justified.

1.0The minimum width adjacent to a road safety barrier and thedesirable minimum for general use. It is also an appropriatewidth when a full width sealed shoulder is to be constructed asa full depth extension of the standard pavement.

1.5 The normal width for a sealed, or partly sealed, shoulder.

2.0 - 2.5For use on higher volume roads, particularly when there is aneed to make periodic provision for vehicles to stop completelyclear of the traffic lanes.

3.0

Normally only used on major urban arterial roads, eg.expressways and motorways. May also be used on highspeed high volume rural and recreational routes where there isa need to make frequent provision for vehicles to stopcompletely clear of the traffic lanes.

NOTES:1. In 100 km of travel a driver would expect to encounter some 4 to 5

stopped vehicles for every 1000 vehicles/hour using the road. Ofthese something less than 5% will have little choice as to the exactlocation of their stop.

2. Shoulders between 0.5 m and 1.5 m do not enable a vehicle to stopclear of traffic lanes. 2.0 m shoulders enable vehicles to stop largelyclear of the traffic lanes.

3. There is evidence that safety does not improve significantly forshoulder greater than 1.5 - 2.0 m wide. Continuous 2.5 m shouldercan therefore normally only be justified only on high volume roadsand where speeds are also high.

4. It is important to provide frequent opportunities for vehicles to stopcompletely clear of the road on all roads with shoulders less than 1.5m, and also on higher volume roads with shoulders less than 2.5 m,by flattening side slopes on some of the lower fills and at thetransitions of cuts and fills.

5. Sealing is sometimes continued beyond the shoulder hinge point anddown the batter slope on the high side to protect the pavement fromingress of water. On floodways, the seal should be continued downthe batter on both sides where no other protection is provided.

Table 6.7: Rural Road Shoulder Width Guidelines

(b) Sealed Shoulder Width

Shoulders on rural roads should be sealed for aminimum width of:• 0.75 m when the predicted design year AADT is

between 500 and 2000, and• 1.5 m when the predicted design year AADT is

greater than 2000.

Wider sealed shoulders are required when provision isto be made for cyclists, see Section 6.3.8.

A full width sealed shoulder should be provided:• adjacent to a lined side drain or kerb,• where a safety barrier is provided adjacent to a

1.0 m wide shoulder,• on the outer shoulder of a superelevated curve,• where a rigid pavement is proposed,• where environmental conditions require it, eg. to

minimise maintenance costs in high rainfall areasand on floodways.


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6.3.3 Multilane Rural Roads, Expressways andMotorways

(a) Multilane Rural RoadsShoulders on dual carriageway rural roads, with twolanes or more in each direction, must be at least:

• 2.0 to 2.5 m wide on the left hand side(nearside), and

• 1.0 m wide on the median side (offside).

Shoulder seal width should be determined in the samemanner as for two-lane two-way rural roads.

(b) Expressway and Motorways

Shoulders on expressways and motorways need to be atthe upper end of the width range, because the high travelspeeds on these roads are usually combined with hightraffic volumes. Shoulder widths for these types of roadshall be:(i) Nearside (Left) shoulder

A minimum of 2.5 m, to allow most smallervehicles to stop clear of the running traffic lane. The desirable width is, however, 3.0 m and thisshould be provided wherever possible.

(ii) Offside (Median) shoulder

1. A minimum of 1.0 m for two-lane one-waycarriageways but greater widths should beprovided wherever possible. 1.5 m is thedesirable minimum width, at least 1.8 m isneeded adjacent to a safety barrier to ensure100% lane capacity and 2.0 m provides shyline clearance.

2. A minimum of 2.0 m for one-waycarriageways with three or more lanes. 3.0 m is, however, the desirable medianshoulder width in these cases and it shouldbe provided wherever possible, to allowvehicles to stop clear of the adjacent trafficlane.

(iii) Clearances to Physical ObstructionsWhere there are physical obstructions adjacent tothe traffic lanes, eg. safety barriers, shoulderwidths for expressways and motorways aredetermined by lane capacity and shy lineclearance considerations. 100% lane capacityclearance must be provided in all circumstancesand shy line clearance provided whereverpossible. Refer to Tables 6.1 and 6.3 for detailsof lane capacity and shy line clearances.

eg. To achieve 100% lane capacity at designspeeds of 100 km/h and higher the shoulderadjacent to a median barrier must be at least1.8 m wide. A 2.0 m shoulder will provideshy line clearance and is the desirableminimum median shoulder width in thesesituations.

6.3.4 Auxiliary LanesThe width of the nearside shoulder adjacent on an auxiliarylane should be:• 1.0 m generally,• 2.0 m adjacent to a safety barrier, and• 3.0 m in merge areas.NOTE: At merges it is important that the shoulder remains

trafficable and a full width sealed shoulder isdesirable in these areas.

6.3.5 On and Off RampsRamp shoulders must be fully sealed and have shoulderwidths of:

• a minimum of 1.0 m and desirably 2.0 m on the lefthand side (nearside), and

• a minimum of 0.5 m, and desirably 1.0 m, on the righthand side (offside).

Ramp shoulder widths may, occasionally, be reduced inspecial circumstances but the total carriageway width shouldnot be less than 5.0 m, and never less than 4.5 m. Refer todrawing TNZ 1/2000/77/7994/1 for the standard geometricdesigns for motorway on and off ramps.

6.3.6 Urban RoadsWhere a 'traffic lanes + shoulders' design is appropriate foran urban road the guidelines for shoulder widths on ruralroads may be applied. However, urban traffic conditions areusually quite different to rural traffic conditions, eg. urbanarterial roads normally have frequently repeated peak hourflows while rural arterial roads normally have fairly consistenttraffic flows. Urban road operating speeds are also usuallywell below free running speeds and design volume/capacityis the most important factor. It is rare, therefore, that a fullwidth, fully paved shoulder can be economically justified asa breakdown lane. Table 6.8 gives guidelines for shoulderwidths on urban roads.

Nominal Shoulder

Width(m)

Situation

0The clearance between a traffic lane and a semi-mountablekerb, eg. a raised median or island, can be zero (0) but aclearance of 0.5 m to any kerb is desirable in all situations.

0.5 - 1.0 The normal offside (median) shoulder for a depressed median.

1.0 The minimum nearside shoulder for general use.

1.5 - 2.5The normal nearside shoulder for use on major urban roads - inurban situations isolated stopped vehicles on a 1.8 m shoulderhave been shown to have a negligible effect on capacity andsafety.

3.0

For use in special cases where the frequency of stoppedvehicles is combined with high traffic volumes and high speeds. This situation is mostly found on arterial outlets to major citieswhere recreational peaks are much ‘flatter’ than urbancommuter peaks, and where traffic volumes just below thosethat can cause considerable reductions in traffic speeds, oftenpersist for long periods.

Table 6.8 Urban Road Shoulder Width Guidelines


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Figure 6.6: Recommended Sealed Shoulder Widthsto Safely Accommodate Cyclists

Source: WA

Figure 6.8: Development of a Longitudinal Crownon a One-way Road Carriageway

Source: WA

Figure 6.7: Two-lane Two-way Road Typical Crossfall Details

6.3.8 CyclistsWhere there is an identified need to provide for cyclistsFigure 6.6 should be used to determine the sealed shoulderwidth needed to accommodate them in a safe manner. Referalso to Section 6.2.9: Cycle Lanes.

6.4 Crossfall6.4.1 GeneralCrossfall is the slope of the carriageway surface, as measurednormal to the road alignment. Its main purpose is to facilitatepavement drainage.

On straight sections of road the pavement crossfall usuallyslopes downwards from either the centreline or the median.However, an inward sloping crossfall, or one-way crossfallmay occasionally be needed for some special alignment,drainage or side slope situations, eg. a short straight betweentwo horizontal curves in the same direction.

On curved sections of road the carriageway normally slopesupwards from the centreline or median, to help counteract thecentrifugal forces on vehicles travelling around the curve.This form of crossfall is known as superelevation.

Changes from one crossfall to another must be transitionedover a length to satisfy the rate of rotation and relative graderequirements given in Section 4.5: Superelevation.

Sufficient dimensions, levels, cross sections and/or profilesmust be provided on road construction drawings to enable thedesign to be accurately reproduced in the field.

6.4.2 Pavement CrossfallOn two-way carriageways the traffic lane(s) are usuallyconfigured in cross section to form an inverted ‘V’, whichusually gets rounded at its highest point, or crown, during thepavement construction process.

On wide dual carriageway roads it may sometimes benecessary to longitudinally crown a one-way carriageway, sothat one, or two, of the traffic lanes drain towards the median.This minimises the depth of water flows on the pavementsurface by reducing flow path lengths, which helps reduce thepotential for aquaplaning. Figure 6.8 illustrates the desirableand undesirable methods of developing a longitudinal crownon a one-way carriageway.

The crossfall required on straight sections of road, whichshould be as flat as possible, is determined by the surfacewater drainage requirements of the pavement and its surfacetexture. For a given crossfall the smoother the surface themore efficient it is in shedding water. This feature must,however, not override the requirements to maintain at leastthe design friction requirements between vehicle tyres and thepavement, for safety reasons.


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6.4.2 Shoulder CrossfallOn straight sections of road sealed shoulders should normallyhave the same crossfall as the adjacent traffic lane. In areaswhere heavy rains are expected, shoulder crossfalls may bemade 1 to 2% steeper than those used for the traffic lanes, toassist in draining the pavement surface quickly.

On horizontal curves the shoulders should be superelevatedto the same crossfall as the adjacent traffic lane.

Typical crossfalls for various types of pavement surface areshown in Table 6.9.

Road SurfaceTraffic Lane

(%)Shoulder

(%)

Cement Concrete 2.0 - 3.0 2.0 - 4.0

Asphaltic Concrete 2.5 - 30 2.5 - 4.0

Chip Seal 3.0 - 4.0 3.0 - 4.0

Unsealed 3.5 - 4.0 4.0 - 5.0

Table 6.9: Typical Pavement Crossfalls

6.4.3 Crossfall in Urban AreasIn urban areas there are many controls which may forcedepartures from the crossfall values shown in Table 6.9.

Where it is necessary to increase crossfalls the maximumsustained crossfall should not exceed 4%. A local increase toa maximum of 6.0% may be acceptable, but only in extremecases because the stability of high vehicles becomes aproblem on crossfalls greater than this as does the clearancesto poles, signs, etc.

6.4.4 Median CrossfallThe crossfall of the adjacent carriageway should be carriedacross the median shoulder.

Paved medians, including those bordered by kerbs, should becrowned at the centre and generally follow the crossfall of theadjacent pavement.

Medians up to about 9 m wide should be approximately levelor follow the crossfall of the road.

Medians greater than 9 m wide should be sloped downwardfrom the adjoining carriageway shoulders to form a shallowvalley in the centre. The median side slope should be #1:10with #1:20 being preferred. Side slopes as steep as 1:6 mayonly be used in exceptional cases when necessary fordrainage, stage construction, etc.

At intersections the median cross slope must match the slopeof the road through the intersection and should not be greaterthan 6.0%. Crossfall should be provided on right turn lanesin a manner that ensures the lane is adequately drained andthe amount of water at the median nose area is minimised.

6.4.5 Footpath CrossfallIt is usual to slope the footpath towards the road, so thatstormwater does not drain on to adjoining properties. Wherethis cannot be achieved drainage onto adjacent propertiesmust be arranged with the property owners. This is notusually an issue in rural areas.

Footpath design requires consideration of several factors:

• Drainage across the footpath• Pedestrian requirements for a walkway• Use by wheel chair bound people• Requirements for sight impaired people.

A 3.0% crossfall is best for drainage reasons but this conflictswith the need to accommodate wheel chairs. For wheel chairsa maximum crossfall of 2.5% should be used but 2.0% ispreferred. In these circumstances, care is required to ensurethat water does not accumulate on the walkway or become ahazard to people with disabilities. Figure 6.9 illustratestypical footpath cross section design details.

Source: Queensland MRFigure 6.9: Footpath Crossfalls


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Figure 6.10: Typical Clear Zone Cross Section Details

6.5 The Clear Zone6.5.1 GeneralIn spite of careful attention to the geometric design of roadsand the application of safety/guidance devices, such aspavement markings and traffic signs, vehicles do occasionallyrun off the road. There are a number of reasons why thishappens, including:

• Driver fatigue or inattention• Excessive speed• Driving under influence of alcohol or drugs• Collision avoidance• Roadway conditions such as ice, snow or rain• Vehicle component failure• Poor visibility.

Studies have indicated that, on high speed roads, a cleartraversable width of about 9 m from the edge of the trafficlane allows the drivers of about 80 percent of vehicles thatrun off the road to regain control with minimum damage toboth vehicles and occupants. This area is termed the 'clearzone' and it must have a slope #1:6 and be kept free ofobstacles or contain only objects which will collapse or breakaway on impact without significantly damaging errantvehicles.

An appropriate minimum clear zone width is desirable on allstate highways and should be provided wherever practicable.On motorways and expressways the desirable minimum clearzone width is 9 m. This clear zone width may be difficult tojustify for engineering, environmental and/or economicreasons on two-lane two-way roads, because of lower speedsand traffic volumes. The minimum desirable clear zone inthese situations is obtained from Table 6.10 or Figure 6.12,with adjustments made for horizontal curvature, gradient andcut/fill side slope.

Although the clear zone concept is normally applied tounkerbed rural roads an appropriate speed related clear zoneshould also be provided in urban areas, especially on newconstruction works. Figure 6.10 shows cross sections detailsfor three typical clear zone situations.

Clear zone width is measured from the outside edge of theadjacent traffic lane and includes the any adjacent auxiliarytraffic lanes, shoulders, medians, verges, footpaths andtraversable batters. This width is related to site-specificconditions such as predicted traffic volume, traffic speed,road geometry, side slope, weather, development adjacent tothe road, and environmental conditions. It also applies toboth sides of the vehicle, including the right hand or off sideof the vehicle in dual carriageway median lane situations.

Obstacles located in the clear zone should be removed,relocated, made breakaway, or shielded by guardrail or crashcushions.

6.5.2 Clear Zone Requirements(a) To be regarded as part of the clear zone the roadside

area:• should be traversable and relatively flat, ie. side

slopes must be #1:6,

• side slopes must not be steeper than 1:4 onembankments and 1:3 in cuttings,

• side slope must have changes rounded in amanner that ensures all wheels of an errantvehicle remain in contact with the ground, toassist the driver to regain control of their vehicle,and

• must be clear of large, fixed objects such as treeswith an ultimate trunk diameter greater than 100mm, structure support piers, culvert headwalls,large solid, ie. non frangible, sign supportstructures, non traversable gutters and barriers,etc, because colliding with these would causeunacceptable rapid deceleration rates to theoccupants of an impacting vehicle. Only objectswhich will collapse, or break away on impact,should be located in the clear zone, to ensureminimal damage to an errant vehicle and itsoccupants.


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Source: AASHTO Figure 6.11: Fill Slope Parameters

Table 6.10 and Figure 6.12 show the lateral clearance,or clear zone width, required on a straight level sectionof road in respect to design/operating speed and AADT.

Where it is not possible to provide an adequate clearzone free of non-frangible obstacles the need for asafety barrier should be investigated. The provision ofa clear zone is, however, often better practice than theerection of a safety barrier, due to the length of barriergenerally required.

NOTE: Clear zone widths obtained from Table6.10 and Figure 6.12 represent areasonable measure of the degree ofsafety considered appropriate for statehighways. The widths are approximateonly and both the table and diagram mustnot be used to infer a degree of accuracythat does not exist, ie. clear zone widthsshould be rounded to the nearest 0.5 m.

* Where a site specific investigation indicates a high probability of continuing accidents, or such occurrences are indicated by accident history, clear zonedistances greater than 9 metres may the designer may be provided, as indicated. Clear zones may be limited to 9 metres for practicality and to provide aconsistent roadway template if previous experience with similar projects or designs indicates satisfactory performance.

* * Since recovery is less likely on unshielded traversable 1 :3 slopes, fixed objects should not be present in the vicinity of the toes of these slopes. Recoveryof high-speed vehicles that encroach beyond the edge of the shoulder may be expected to occur beyond the toe of slope. Determination of the width of therecovery area at the toe of slope should take into consideration the road reserve area, environmental concerns, economic factors, safety needs, and accidenthistories. Also, the distance between the edge of the traffic lane and the beginning of the 1:3 slope should influence the recovery area provided at the toeof slope. While the application may be limited by several factors, the fill slope parameters which may enter into determining a maximum desirable recoveryarea are illustrated in Figure 6.11.

Source: AASHTO Table 6.10: Clear Zone Width Required on a Straight Level Section of Road


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Source: AASHTO Figure 6.12: Clear Zone Width Required on a Straight Level Section of Road


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6.5.3 Clear Zone Width Adjustments(a) General

The clear zone requirement at any point on a road isdetermined by its horizontal and vertical alignment andcross section side slope features.

Adjustments must be made for horizontal curvature,gradient and side slope. The largest of the adjusteddistance is the clear zone width required.

Fill embankment slopes are classified as recoverable,non-recoverable or critical. These classifications are:• Recoverable Slopes (Traversable)

Fill embankment slopes of less than 1:4 aregenerally considered recoverable. Arecoverable slope $1:6 enables drivers to retain,or regain, control of their vehicles. Vehicles onrecoverable slopes can generally be stopped orslowed down and returned to the carriageway.

• Non-recoverable Slopes (Traversable)

Fill embankment slopes in the range 1:4 and 1:3are considered non-recoverable slopes. Vehiclescan generally be slowed down and stopped, butthey are normally unable to be easily returned tothe carriageway. An errant vehicle will usuallyreach the bottom of this type of slope and a clearrun-out area at the base of the slope is, therefore,desirable.

• Critical Slopes (Non-Traversable)

Fill embankment slopes steeper than 1:3 areconsidered critical slopes because vehiclestraversing them are likely to overturn. If acritical slope starts within the clear zone theprovision of a safety barrier should beconsidered.

(b) Adjustment for Road Alignment(Horizontal Curvature and Gradient)

Horizontal curvature and gradient can significantlyaffect roadside encroachment rates. American researchhas shown that the Effective Traffic Volume (ETV) canbe used to relate encroachment frequency with roadalignment.

ETV is defined as the traffic volume on a straight flatsection of road that is equivalent to the traffic volume ona section of road with horizontal curvature and/or gradesand is calculated by the following formula:

EVT ' K x AADTWhere:

AADT = AADT in Design YearK = Volume Adjustment Factor.

K is obtained from Figure 6.14, or Figure 6.15 ,using theEncroachment Frequency Adjustment Factor, M, fromFigure 6.13.

The clear zone width for a straight level road forAADT = ETV is then obtained from Table 6.10 or Figure6.12.

(c) Side Slope Adjustment(i) General

A roadside slope affects the ability of a driver tomanoeuvre an errant vehicle back onto the carriageway.When the roadside slopes upward the encroachmentdistance is reduced because of the beneficial effect ofthe slope on braking and steering. Conversely, adownward slope will increase the encroachment distanceand can increase the severity of the encroachment, eg.cause a rollover.(ii) Effective Clear Zone Width

A variable fill embankment slope, with a relatively flatrecovery area immediately adjacent to the road followedby a steeper side slope, can sometimes be used to lessenthe amount of land, and also fill material, required for anew road, eg. if an adequate clear zone width isprovided by the flatter slope the steeper slope may becritical or non-traversable.

Clear zone widths for variable fill embankment slopesranging from level to 1:4 may be averaged to give acomposite clear zone width.

Slopes which change from negative to positive cannotbe averaged and should be treated as roadside ditchesand analysed for traversability, refer to Section 7.2:Ditches and Back Slopes for details.

Section 3.3.4 of the AASHTO Roadside Design Guidecontains several worked examples showing how theclear zone width can be determined for variable slopes.


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(a) (b)

Figure 6.14: Traffic Volume Adjustment Factor, K, for Two-lane Two-way Roads

Figure 6.15: Traffic Volume Adjustment Factor, K, for Dual Carriageway Roads

(b)(a)

Figure 6.13: Encroachment Adjustment Factor M


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6.5.4 Roadside Safety Barrier WarrantRoadside safety barrier warrants are typically based on asubjective analysis of roadside elements or conditions. Thetwo main factors normally used in determining the need for asafety barrier on an embankment are the height of the fill andits side slope. However, the probability of a roadsideencroachment, which is related to the current AADT on theroad, also needs to be considered. A warrant for use as aguide for the provision of roadside safety barriers on statehighway fill embankments is given in Figure 6.16.

6.6 Medians6.6.1 GeneralMedians separate the carriageways for traffic travelling inopposite directions on dual carriageway roads and, formaximum efficiency, they should be highly visible night andday and should also contrast with the traffic lanes. Mediansare used to:

• significantly reduce the risk of collisions by separating opposing traffic streams,

• provide a recovery area for out-of-control vehicles,• provide a stopping area in case of emergencies,• minimise the effects of headlight glare• accommodate safety barriers and glare screening.• provide width for future lanes,• improve capacity by restricting access to property and

minor side streets,• prevent indiscriminate u-turn movements,• provide space for speed changes and storage of

right-turning / 'U'-turning vehicles and restrict thesemovements to signalised intersections and/or rightturn bays,

• provide a safety refuge for pedestrians,• collect stormwater run-off from the road and carry it

to drainage system, and• provide an open green space in urban areas.

Medians are highly desirable on arterial roads with four ormore traffic lanes and may have a depressed or raised form,or be made flush with the surface of the carriageways

Median width is measured between the edges of opposingtraffic lanes, including the adjacent offside (right hand)shoulders, if any. Median widths range from a minimum of1.2 m in urban areas to 25 m or more in rural areas

Economic factors generally limit median width. Constructionand maintenance costs increase in proportion to median widthbut the additional cost may not be appreciable compared withthe cost of the road as a whole, and may be justified in viewof the benefits derived. Medians should, however, be madeas wide as possible while also being in balance with othercomponents of the road cross section. As far traffic operationis concerned a freedom of operation, in the sense of physicaland psychological separation from opposing traffic, isachieved when median widths are about 12 m or greater. Atsuch widths the carriageways are truly divided, the noise andair pressure of opposing traffic is not noticeable and the glarefrom headlights at night is greatly reduced. With widths of18 m, or more, a median can be pleasingly landscaped in apark-like manner. Planting can be used to achieve this effectbut it must not compromise the clear zone requirements.

Where possible, median widths should be such that safetybarriers are not warranted. However, consideration must begiven to the need for a safety barrier when determiningmedian widths and Figure 6.17 contains the warrant for theprovision of median barriers on dual carriageway statehighways. This warrant has been developed from researchstudies and analysis of the limited data available on cross-median accidents and, in the absence of site specific or morerecent data, an explicit level of accuracy should not beimplied.

Figure 6.16: State Highway Fill EmbankmentSafety Barrier Warrant


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The performance, location and placement requirements forroadside safety barriers erected on state highways are detailedin Section 7.3 of this manual, Longitudinal Road SafetyBarriers. Details of the location and placement of road safetybarriers within medians, and the method for determining theperformance levels required for these barriers are given inSubsection 7.3.12: Median Barriers.

Wider medians should be provided on roads with at-gradeintersections. The median should be wide enough toadequately shelter the design vehicle(s) crossing atintersections as well as allowing for safe manoeuvres atcommercial and/or private accesses. These intersections mayneed to be controlled, for safety reasons, but the clearancetimes required for vehicles to cross a wide median may leadto inefficient traffic signal operation.

Where road reserve width is restricted, wide medians may notbe justified, if they have to be provided at the expense ofnarrowed verge areas. An adequate verge width is requiredto provide a buffer between private development along a roadand the carriageway, particularly where zoning is limited ornon-existent. The verge area must provide space forfootpaths, traffic signs, utility services, parking, drainagechannels, structures, reasonable clear zones with proper sideslopes, and any retained natural growth. Narrowing theseareas may tend to the development of obstacles andhindrances similar to those a median is designed to avoid.

A depressed median configuration should normally be usedon rural roads for more efficient drainage reasons and the sideslopes should be traversable. Median side slopes:

• should preferably be #1:20,• should not exceed 1:10, particularly when a median

barrier is installed, and• must not exceed 1:6.

NOTE: The appropriate clear zone width, and/ormedian barrier location, must also beachieved in all situations.

Steeper slopes, ie. up to 1:4, may, however, be considered inurban areas and on rural roads with independently aligned andgraded carriageways.

In general, depressed medians should be kept clear ofobstructions within the clear zone requirements of the roadand the use of head walls, unprotected culvert openings, solidsign foundations, non-frangible sign posts and light polesshould be avoided. Where longitudinal culverts are required,eg. under cross overs, the ends facing traffic should be slopedat 1:20 (preferably), no steeper than 1:6, and provided withtraversable safety grates. All other drainage inlets should bedesigned with their tops flush with the ground.

Raised medians should only be used on arterial roads in urbanareas, particularly where it is desirable to regulate right-turnmovements. They can also be used where the median is to beplanted, particularly on relatively narrow medians. Carefulconsideration must be given to the location and type ofplanting in these situations because it can create problems formaintenance activities and larger plants, such as trees, cancause visual obstructions for the drivers of turning vehicles.

The use of painted, or flush, medians on urban roads hasbecome a common practice and widths of 3.0 to 4.8 m willusually provide an optimum design in these situations. Flushmedians offer several advantages when compared to multi-lane roads without medians, including:

• reduced travel time,• improved capacity, ie. remove right turning vehicles

from the traffic lanes,• reduced accident frequency, particularly of the

rear-end type, and• public preference, both from drivers and owners of

abutting properties.

Flush medians may also be used on urban expressways wherespeeds are 70 km/h or less but median safety barriers might berequired in some cases. The median area should be slightlycrowned or depressed. The crowned type eliminates the needfor collecting drainage water in the median. The slightlydepressed type is, however, generally preferable and a crossslope of about 4 percent, or a minor steepening of theroadway crossfall, should be used.

In general medians should be designed to ensure that they areas maintenance free as possible. This will minimise theamount of time that maintenance personnel will be requiredto spend on the median thereby reducing their exposure totraffic hazards. Planting should consists of 'frangible' specieswith ultimate trunk diameters no greater than 100 mm, unlessthey are outside the clear zone and/or located behind an

Source: AASHTOFigure 6.17: State Highway Median Barrier Warrant


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appropriate safety barrier. Landscaping design and speciesselection will depend on the specific circumstances andrequires specialist input. Features in medians that limithorizontal sight distance on curves should normally belocated such that adequate sight distance is achieved. Theoffset needed to achieve this is illustrated on the diagram inFigure 6.18 which shows a median landscape treatment forright hand curves.

Where street lighting is not provided headlight glare acrossthe median can be a nuisance, particularly where the road hasrelatively sharp curvature. Under these conditions some formof antiglare treatment should be considered in the mediandesign, usually as part of a safety barrier installation.

6.6.2 Urban RoadsIn urban areas it is desirable to provide a median on roadswith four or more lanes. For good traffic operation a one-waycarriageway width of more than 9.0 m between kerbs isrequired to allow for adequate two travel lanes plus a sharedparking lane/travel lane. On carriageways less than 9.0 mwide parking and/or breakdowns will restrict traffic flows andreduce capacity. Medians can, however, be provided on roadswith carriageways that have 7.0 m to 9.0 m between kerbswhere:

• mountable kerbs are used and an area outside the kerbis provided to enable vehicles to stop clear of thetravel lanes, or

• the design hourly traffic volumes can beaccommodated in one travel lane when separate turnlanes are provided for significant turning trafficvolumes.

At signalised intersections medians need to be wide enoughto accommodate signal posts, lanterns and servicing facilities.The desirable minimum median width to accommodate thesefeatures is 2.4 m, and this also allows for maintenance ladderspread. Where there are three lanes or less at the stop line, aminimum median width of 1.2 m may be used, provided thatall intersection movements can be adequately controlled byoverhead lanterns on mast arms.

6.6.3 Rural RoadsA median should always be provided when a rural road iswidened to four lanes or more, or is constructed initially as adual carriageway road.

Research has shown that the minimum median width requiredto adequately separate traffic on high speed dual carriagewayroads, without the use of safety barriers, is about 15 m.Where 15 m cannot be achieved the minimum median widthshall be determined by the largest of the clear zones requiredfor either carriageway. The clear zone in these situationsshall be measured from the outside edge of the median trafficlane to the outside edge of the shoulder of the oppositecarriageway. Where this minimum width cannot be achieveda median safety barrier will probably be required, refer toFigure 6.17 for the warrant for the provision of medianbarriers on dual carriageway state highways.

The absolute minimum median width for a rural road, andwhich may only be used as a last resort in situations where thewidth of the road reserve is very restricted, is a rigid roadsafety barrier with two minimum width median shoulders.Refer to Section 6.3.3 for median shoulder widthrequirements.

Rural roads will sometimes have independently aligned andgraded carriageways. Special attention needs to be given tothe carriageway relationship in these cases, to minimise theeffects of headlight glare. The median width at any pointmust also never be less than the minimum clear zonecalculated by the method described in the precedingparagraph. If the minimum width cannot be achieved theneed for a median safety barrier must be investigated.

Where it is necessary to provide at-grade cross-median accessfor turning semi-trailers and the like, a wider median willusually be required to ensure that turning vehicles aresheltered from through traffic. The minimum median widthin these cases is governed by the length of the design vehicleexpected to use the facility. Local widening only may beconsidered where it is uneconomic to maintain a wide medianover the entire length of road.

Where the additional traffic lanes are anticipated in the futureit is desirable to allow for them by providing extra width inthe initial median. The ultimate median should, however, notbe less than the desirable minimum width of 15 m. Theadvantages of providing of addition traffic lanes by wideninginto the median are:

• minimum traffic disruption during construction of thewidening,

• minimum interference with roadside furniture,drainage installations and environmental protectiondevices,

• prevention of further environmental damage duringconstruction of the widening, and

• avoiding disturbing cut batters (particularly importantin potentially unstable or erodible country).

There will, however, be circumstances where widening on theoutside of existing pavements will be the appropriate solution,including where:

• existing ramps have to be remodelled to suit currentstandards,

• the existing median is too narrow to accommodate theadditional lanes needed and retain a sufficient width,

• there is little disruption to the existing drainage andother infrastructure,

• the existing outer lane pavement has less liferemaining than the inner lane and a new outer lane,which will carry most of the heavier loaded slowervehicles, will allow additional life to be achieved forthe pavement as a whole, and

• adjoining sections at the start and end of a relativelyshort project have to be matched.

The decision on how to provide for future widening requirescareful consideration of all of the factors involved with thedesign of the road in question and adopting the solution thatprovides the best answer for those particular circumstances.Figure 6.19 illustrates typical rural median details.


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Figure 6.18: Typical Urban Arterial Median Cross Section Details

6.6.4 Expressways and MotorwaysThe requirements for rural road medians described in Section6.6.3 above also apply to expressways and motorways.

If road widening is likely in the future it should be applied onthe median side of the carriageway. The width of the medianin the initial construction stage should therefore be such that,after the widening is completed, an acceptable median widthis retained. Desirably, this width should be at least 15 m toprovide the safest situation.

In constrained urban situations, however, narrower medianswith safety barriers similar to those used on urban arterialsmay be more appropriate. The width available for the medianwill dictate the type of barrier that can be used, based on lanecapacity preservation, shy line and barrier deflectionrequirements. Some typical arterial median cross sections thathave been used in restricted urban situations are illustrated inFigure 6.18.

The absolute minimum median width for a dual carriagewayexpressway or motorway with a design speed of $100 km/his a rigid barrier plus a clearance from the face of the barrierto the edge of the adjacent traffic lane of 2.0 m for a two-laneone-way carriageway, and 3.0 m for a three or more lane one-way carriageway.

This provides the required shy line clearance and ensures100% traffic capacity for the median traffic lane. Refer toTables 6.1, 6.2 and 6.3 for more details on shy line and trafficlane capacity clearances.

The extensive use of concrete median barriers is not apreferred option, on environmental grounds, and this aspectneeds careful consideration when determining median widths.

6.6.5 Raised MediansRaised medians should only be used in urban areas and wheredesign speeds are is 70 km/h or less.

A lateral clearance of at least 0.5 m must be provided fromthe edge of the traffic lane to the face of the kerb of a raisedmedian in unlit areas. In lit areas no lateral clearance isnormally required from the edge of a traffic lane to a raisedmedian. Where the median kerb incorporates a drainagechannel, the channel must be located outside the traffic lane.

If the design speed is greater than 70 km/h a minimum lateralclearance of 1.0 m from the edge of the traffic lane to the faceof the kerb or safety barrier must be provided in unlit areas.In lit areas this clearance may be reduced to 0.5 m for shortlengths of road, ie. no more than about 500 m, where the roadreserve width is restricted.


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Figure 6.19: Typical Rural Median Cross Section Details

Documents

State highway geometric design manual cross section